In the first study of its kind, researchers have linked specific vitamins and nutrients in the diet with cognitive performance and the risk of Alzheimer’s disease.

The research, published in the journal Neurology, showed that people with healthier diets — rich in omega-3 fatty acids and a variety of vitamins — had bigger brains and better cognitive function than those whose diets were unhealthier on the whole.

Many previous surveys of people have found that those who report diets high in vitamin E and omega-3 fatty acids have slower rates of cognitive decline, compared with people whose diets are lower in these nutrients. But when researchers have conducted randomized trials with elderly patients, giving specific supplements to some and placebos to others, the association between the nutrients and intellectual abilities like memory, language, reasoning and planning fell apart.

Part of the problem, says Gene Bowman, a nutritional epidemiologist at Oregon Health & Science University, is that the participants in these observational studies were asked to remember what they ate by answering questionnaires. But if the primary outcome was to measure their cognitive abilities, including memory and recall, the studies were clearly flawed — how reliable could the volunteers’ answers be, if they were suffering from cognitive decline?

So Bowman and his colleagues came up with a way to address that fault. They conducted the first study to use an objective measure of dietary nutrient content: by measuring levels in the blood. The study involved 104 people, who were elderly (average age 87) but relatively healthy. Researchers analyzed their blood for a variety of vitamins and nutrients, including vitamins B, C, D and E, saturated fat, carotenoids, omega-3 fatty acids, cholesterol and trans fats. Then they compared those levels to participants’ performance on cognitive tests as well as MRI scans looking at differences in the size of certain brain structures related to Alzheimer’s.

The team found that people who had higher blood levels of vitamins B, C, D and E and omega-3 fatty acids scored higher on the mental-function tests, including attention tasks and visual and spatial skills, than those with lower levels of these nutrients. People who had higher levels of trans fats in their blood, by contrast, scored lower on these tests; they took more time overall to complete the tests and had more trouble with memory and language skills.

Omega-3s and vitamin D are found primarily in fish, while vitamins B, C and E are high in fruits and vegetables. Trans fats come largely from packaged, fried, frozen and fast foods, along with baked goods and margarine spreads.

When the scientists took into account known risk factors for Alzheimer’s disease — age, gender and genetic mutations — they found that these factors were responsible for 46% of the difference in participants’ cognitive scores. In other words, people who were older and had the APOE4 gene mutation that is a risk factor for Alzheimer’s, were more likely to score lower on cognitive tests than younger participants who didn’t have the genetic mutation.

When Bowman’s group added in the effect of participants’ diet, however, they found that their nutritional profiles explained another 17% of the variation in cognitive scores.

The researchers then looked at the size of specific brain structures on the MRI. Known Alzheimer’s risk factors accounted for about 40% of the difference in cognitive scores between those with normal-size brains and those with smaller brain volumes, while diet explained another 37% of the variance. Brain size normally shrinks with age, but with Alzheimer’s disease, that shrinkage is accelerated — a sign that the condition is getting worse. “That means that diet, plus known risk factors, explained a total of 76% of the variance,” says Bowman. “That tells us that imaging and structural changes in the brain may be very sensitive to dietary intake. So imaging may actually have a greater power to detect relationships between diet and cognitive decline than tests of mental skills. That’s quite remarkable.”

It’s possible, then, that clinicians may someday use brain scans to identify brain-size changes — and cognitive decline — attributable to deficiencies in certain nutrients or supplements. This is the first study to objectively measure the potential association between diet and brain aging, however, so further research is needed to confirm the connection.

It’s also the first study to capture the combined effect of a variety of nutrients on the brain. Previous studies that have focused only on single nutrients may have failed to detect an effect because nutrients may work together to protect brain functions from the effects of aging or disease.

The study also opens up the possibility that we may be able to use individualized dietary treatments to enhance whatever aspects of brain function — memory, attention or higher learning — are declining fastest. For example, Bowman found that participants in the study who had higher levels of vitamins B, C, D and E did not have problems with memory, but did show trouble with attention and visual-spatial tasks, while those with higher levels of carotenoids (found in carrots and dark leafy green vegetables) also showed fewer problems with memory.

“It’s a platform for individualized nutritional therapy,” he says. “We’re already seeing different nutritional patterns associated with different cognitive domains, so not only does this help us understand the role of diet in brain aging, but also how we might individualize nutritional therapy to enhance brain function as we age.”